(1) Field of the Invention
The invention relates to a method of metal via formation, and more particularly, to a method of forming a spin-on-glass intermetal dielectric layer using large tilt angle ion implantation to prevent poisoned via metallurgy in the fabrication of integrated circuits.
(2) Description of the Prior Art
The spin-on-glass materials have been used for planarization of integrated circuits. The material to be applied is thoroughly mixed in a suitable solvent. The spin-on-glass material suspended in the vehicle or solvent is deposited onto the semiconductor wafer surface and uniformly spread thereover by the action of spinning the wafer. The material fills the indentations in the integrated circuit wafer surface; that is planarization. Most of the vehicle or solvent is driven off by a low temperature baking step often followed by vacuum degassing. Other coatings of the spin-on-glass material are applied, baked and vacuum degassed until the desired spin-on-glass layer is formed.
The final step in the making of the spin-on-glass layer is curing. Curing is a high temperature heating step to cause the breakdown of the silicate or siloxane material to a silicon dioxide like cross linked material. A cured spin-on-glass process is described in U.S. Pat. No. 5,003,062 to Yen.
In the conventional etchback process as taught by Yen U.S. Pat. No. 5,003,062, the spin-on-glass layer is etched back resulting in a poisoned via metallurgy, such as aluminum, caused by outgassing. Referring to FIG. 1, there is shown a partially completed integrated circuit in which gate electrodes 12 have been formed on a silicon substrate 10. A first insulating layer 14 is deposited over the gate electrodes 12. A first metal layer 16, typically aluminum or an aluminum alloy, is deposited and patterned. Then the intermetal dielectric layer is formed, consisting of a spin-on-glass layer 20 sandwiched between, for example, silicon dioxide layers 18 and 22. When a via opening 24 is made through the intermetal dielectric layer, leaving a portion of the spin-on-glass exposed, there may be outgassing of water from the spin-on-glass layer. This water reacts with the aluminum causing corrosion of the aluminum. When etchback is not used, poisoned via metallurgy occurs especially in small via sizes.
U.S. Pat. No. 5,219,792 to Kim et al teaches covering the spin-on-glass edge within the via opening with a silicon oxide layer to prevent outgassing. In U.S. Pat. No. 5,393,702 to the same inventor, a process is described for forming a silicon nitride cap on a spin-on-glass layer within a via opening to prevent outgassing.
In their paper, "Modification Effects in Ion-Implanted SiO2 Spin-on-Glass," by N. Moriya, Y. Shacham-Diamond, and R. Kalish, J. Electrochem Soc., Vol. 140, No. 5, May 1993, pp. 1442-1449, the authors disclose an ion implantation process to modify the spin-on-glass layer. There is no concept of how to overcome the poisoned via problem mentioned in this article. A major disadvantage of this process is that thick portions of the spin-on-glass layer cannot be affected in depth by the ion implantation. If a via opening is made where the spin-on-glass layer is thick, the poisoned via problem will still exist.